Electrostatic clamps or chucks (ESCs) are commonly implemented to hold workpieces in plasma-based or vacuum-based semiconductor processes such as etching, CVD, and ion implantation, etc. Capabilities of the ESCs, including non-edge exclusion and workpiece temperature control, have proven to be quite valuable in processing semiconductor substrates, such as silicon wafers. A typical ESC, for example, comprises a dielectric layer positioned over a conductive electrode, wherein the semiconductor wafer or workpiece is placed on a surface of the ESC (e.g., the workpiece is placed on a surface of the dielectric layer). During semiconductor processing (e.g., ion implantation), a clamping voltage is typically applied between the workpiece and the electrode, wherein the workpiece is clamped against the chuck surface by electrostatic forces.
De-clamping or un-sticking the workpiece from the chuck surface, however, is a concern in many ESC applications. For example, after the clamping voltage is turned off, the workpiece typically “sticks” to the chuck surface for a considerable amount of time, wherein the workpiece cannot be removed by typical workpiece lifting mechanisms (e.g., pins extending through the ESC which are configured to lift the workpiece from the surface of the dielectric layer). This workpiece de-clamping problem can reduce the throughput of the process. It is believed that the de-clamping problem occurs when residual charges induced by the clamping voltage remain on the dielectric layer or on a surface of the workpiece, therein leading to an undesirable electric field and clamping force. According to a charge migration model, residual charges are caused by charge migration and accumulation during clamping, wherein the charges accumulate at the dielectric surface and/or workpiece backside (e.g., when the workpiece surface comprises an insulating layer).
An RC time constant, for example, can be used to characterize the charge/discharge times which correspond to an amount of time typically required to respectively clamp or de-clamp the workpiece. Conventionally, this time constant is determined by the product of a volume resistance of the dielectric layer and a gap capacitance between the wafer and dielectric surfaces, i.e.,
                    RC        =                                            R              die                        ⁢                          C              gap                                =                                    ρ              ⁡                              (                dielectric                )                                      ⁢                          ɛ              0                        ⁢                          ɛ              r                        ⁢                                          d                ⁡                                  (                  dielectric                  )                                            gap                                                          (        1        )            where Rdie is the resistance of the dielectric layer, Cgap is the capacitance of the gap between the wafer and the chuck surface, ρ(dielectric) is the volume resistivity of the dielectric layer, ∈0 is the free space permittivity, ∈r is the dielectric constant of the gap, d(dielectric) is the thickness of the dielectric layer, and gap is the distance between the dielectric and workpiece surfaces. Thus, depending on the dielectric chosen, the gap, and other factors, the clamping and de-clamping times can be significantly long. A variety of techniques have been conventionally used for reducing workpiece de-clamping problems encountered in the use of ESCs. For example, one conventional technique involves applying a reversal voltage before the workpiece is removed from the ESC, therein eliminating a residual attractive force. This reversal voltage, however, is typically 1.5 to 2 times higher than the clamping voltage, and the de-clamping time is still typically quite large.
Another conventional technique involves providing a low-frequency sinusoidal or square-wave AC clamping voltages to a plurality of electrodes in order to produce wave fields of controlled amplitude and phase. However, in utilizing a sinusoidal AC clamping voltage, the voltage applied to the ESC oscillates between positive and negative voltage, thus passing through zero volts during the oscillation. When passing through zero volts, the clamping force applied to the workpiece is diminished, and a deleterious vibration or “fluttering” of the workpiece with respect to the ESC can be observed, wherein the workpiece can potentially separate from the ESC. In order to alleviate some of the vibration, multiple electrodes have been utilized, wherein a phase clamping voltage applied to each of the electrodes is offset by an amount proportionate to the number of phases provided.
Such a technique is disclosed in U.S. Pat. No. 5,452,177, issued Sep. 19, 1995 to Frutiger, wherein six electrodes are provided. The electrodes disclosed by Frutiger are symmetrically disposed with respect to the center of a clamping surface of a platen, thus defining pairs of electrodes. The voltages applied to the two electrodes of each pair are one-half cycle, or 180 degrees, out of phase with one another. As such, a time exists when the polarity of each pair crosses zero volts, and each pair of electrodes respectively loses clamping force. To alleviate this loss of clamping force, Frutiger further offsets each pair of electrodes by 120 degrees from each other, thus providing two pairs of electrodes that have non-zero voltages applied thereto at the time when the remaining pair crosses zero volts.
Electrostatic clamping systems comprising an ESC having a plurality of electrodes or poles, however, continue to suffer from deleteriously decreased clamping forces associated with the crossing of zero volts when using an AC waveform, even when the conventional proportional phase shifts are implemented. Conventionally, the phase shifts are determined by equally dividing the phase cycle by the number of electrodes in the ESC. For example, a three-electrode ESC will conventionally shift the phase of AC voltage applied to each electrode by 120 degrees, since 120 degrees is exactly one-third of a full cycle. In the six-electrode ESC discussed previously, the phases of AC voltage differ by 60 degrees (a 180 degree shift for each pair, wherein each pair is further shifted from the other pairs by 120 degrees). Such conventional equally-spaced phase shifting, however, does not take into account various other factors associated with the clamping system, such as the RC time constant associated with the entire system, and insufficient clamping force to the workpiece can continue to exist.
Therefore, a need exists in the art for a method for optimizing electrostatic clamping of workpieces in a multiple-electrode ESC, wherein various properties of the electrostatic clamping system are taken into account when determining a phase shift.